Limiting current region

In these techniques, the concentrations at the electrode do not immediately attain their extreme values after the start of the experiment. Rather, they change with E ox t according to equation (1). While the steepness of the concentration profiles increases with E (forward scan), simultaneously 8 increases in the quiet solution. The latter effect slows down the increase of i with E, and finally (close to the limiting current region) leads to the formation of a peak with a characterishc asymmetric shape. On the reverse scan (after switching the scan direction ad. Ef), products formed in the forward scan can be detected (B, in the case discussed). 

This method is sometimes abbreviated to LSV. In this method, a static indicator electrode (A cm2 in area) is used and its potential is scanned at constant rate v (V s-1) from an initial value ( ) in the positive or negative direction (Fig. 5.18). A typical linear sweep voltammogram is shown in Fig. 5.19. In contrast to DC polar-ography, there is no limiting current region. After reaching a peak, the current decreases again.9 For a reversible reduction process, the peak current ip (A) is expressed by Eq. (5.26), where D and C are the diffusion coefficient (cm2 s 1) and the concentration (mol cm-3) of the electroactive species ... 

A rotating disk electrode (RDE) [7] is used to study electrode reactions, because the mass transfer to and from the electrode can be treated theoretically by hydrodynamics. At the RDE, the solution flows toward the electrode surface as shown in Fig. 5.22, bringing the substances dissolved in it. The current-potential curve at the RDE is S-shaped and has a potential-independent limiting current region, as in Fig. 5.6. The limiting current (A) is expressed by Eq. (5.33), if it is controlled by mass transfer ... 

A potential-step is applied to the OTE to switch its potential from the initial value, at which reaction (9.1) does not occur, to the value of the limiting current region of reaction (9.1), and then the absorbance A(t) of substance B is measured as a function of time t.1 1 If reaction (9.2) does not occur, the A(t)-t relation is expressed by Eq. (9.3).2)... 

An alternative to the potential-step method is to open the electrolytic circuit after keeping the potential for a fixed time at the limiting-current region, and then measure the change in absorbance. 

It may be noted that, when a very large step is applied, the potential will have a value in the so-called limiting-current region. For example, in the case of a reduction, this means that co(0, t) equals zero and kf... 

At the rotating disc electrode, if is large, we obtain in the limiting current region [151]... 

This technique was proposed by Bruckenstein and co-workers [280, 281] and is useful in that the current due to the modulation of the fluid flow is essentially free of any electrode surface-controlled contributions in most cases. Thus, it can be used as an analytical tool to increase sensitivity [282]. Step changes were originally considered but this was later extended to sinusoidal hydrodynamic modulation (SHM) in the limiting current region and then to the region of mixed convective-diffusion/kinetic control [283—287]. If the modulation frequency is o, then the modulation, which is small, can be described by... 

LCEC is a special case of steady-state hydrodynamic chronoamperometry. In LCEC, the concentration changes as the chromatographic zones flow past the detector. The electrode is operated in the limiting current region for the eluted compounds, even though the concentration varies as the zones enter and leave the detector compartment. It is important to note that the volume of solution in the active region of the typical electrochemical detector ( 1 pL or even less) is very small compared to the volume occupied by the typical chromatographic zone ( — 0.1-1 mL). 

All photocurrents from a semiconductor, when measured near the limiting current region, have the type of appearance shown in Fig. 10.5. The events that lead to the production, e.g., of hydrogen from the photodecomposition of water on illumination of, say, p-type InP, are as follows ... 

D. S. Ginley and M. A. Butler, J. Appl. Phys. 48 2019 (1977). Photocurrents in the limiting current region interpreted in terms of Schottky barrier. 

Experimentally, the generator electrode current, /gen, is controlled (galvanostatic control), usually being slowly increased from zero in an anodic or cathodic direction depending on the electrode reaction, and the detector electrode is held at a potential such that all the B reaching it is converted into C, i.e. a potential in the limiting current region for B and C, and passes current /det. 

We started this book with a schematic presentation (Fig. lA) of the current-potential relationship in an electrolytic cell from the region where no current is flowing, in spite of the applied potential, to the region where the current rises exponentially with potential, following an equation such as Eq. 8D and through the limiting current region, where the current has a constant value, determined only by the rate of mass transport to the electrode surface or away from it. 

Fig. UK Evolution of the concentration profile with time near an electrode surface, just after the potential has been stepped to the limiting current region.

The response of a spherical electrode to a potential-step function in the limiting current region is given by... 

Electochemical measurements by Jaenicke and co-workers [56] indicate two possible mechanisms by which superadditivity could arise. In the first, a developer showing irreversible oxidation becomes reversible in the presence of a second developing agent. In the second, when the mixed potential is in the limiting current region of the anodic reaction, superadditivity occurs if the limiting current rises because the number of electrons delivered per molecule of a first developer, for example hydroquinone, is increased by the addition of a second developer, such as Phenidone. This could happen with hydroquinone development in the presence of sulfite because the reaction product, hydroquinone monosulfonate, is a poor developer by itself but in the presence of Phenidone it is activated. 

Figure 22-4 Current-potential curve for electrolysis showing the linear or ohmic region, the onset of polarization, and the limiting cuirent plateau. In the limiting current region, the electrode is said to be completely polarized, since its potential can be changed widely without affecting the current.

Figure 9.3.7 Koutecky-Levich plots at potential where the rate of electron transfer is sufficiently slow to act as a limiting factor, and at E2, where electron transfer is rapid, for example, in the limiting-current region. The slope of both lines is (0.62nFACQDQ v ) ...

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